Known force-measuring devices include different functional parts such as a force-receiver, a force-transmitting mechanism, a force-measuring cell, and in some cases a device for the processing of measurement signals. A force that is to be measured is received by the force-receiver and transmitted by way of the force-transmitting mechanism to the force-measuring cell. The force-measuring cell converts the incoming force into an electrical force measurement signal which corresponds to the force acting on the force-measuring device.
Analogously, in the case of a balance the input force is represented by the weight force of the weighing object, the so-called weighing load acting on the force-receiver which has the form of a weighing pan. This force input is transmitted by the force-transmitting mechanism in the form of a linkage to the force-measuring cell or weighing cell where it is converted into an electrical force measurement signal, such as weighing signal.
The electrical force measurement signal is transmitted to a signal-processing unit which serves to further process the force measurement signal and to generate a corresponding output signal. The output signal is transmitted to an indicator unit and/or to a further processing device, for example a master computer or a system controller.
Force-measuring devices or balances of this kind can be used to weigh individual weighing objects, but they also find application in automated production- and test systems for the weighing of larger quantities of weighing goods. A force-measuring device of this type has to meet high levels of accuracy, reproducibility and stability of the measurements. In addition, the force-measuring device should be, as far as possible, of a simple and cost-effective design.
Known practices that influence factors causing errors in the weighing result have to be measured and appropriately corrected to obtain accurate and stable measurements. For example, a method is described in GB 1495278 for correcting the influence of load-independent parameters, in particular the influence of a temperature which affects the weighing device from the outside. The correction is accomplished by measuring the ambient temperature to which the weighing device is exposed and by generating a corresponding electrical temperature measurement signal. Based on the temperature measurement signal, the force measurement signal is then processed into a temperature-corrected output signal. Using this method, it is also possible to correct time-dependent phenomena, for example the creep effect following an elastic deformation, by means of a time-dependent exponential function.
This known correction method is based on the assumption that the force-measuring device is in a condition where all of its components share the same temperature, for example the temperature of the ambient environment. However, situations often occur in which the different components of the force-measuring device have unequal temperatures. For example, it is possible that additional heat is generated in a force-measuring device during operation, which raises the temperature in some components of the force-measuring device. Consequently, there will be a temperature difference between the operation related temperatures of the components and the ambient temperature to which the device is exposed.
In general, temperature differences of this kind will affect the measurements of the force-measuring device, for example the measurements of the input force or input load acting on the device. The aim is therefore to correct the influence of these temperature effects as completely as possible, particularly in force-measuring devices that have to meet high standards of accuracy and stability.
Known devices offer different solutions for correcting the effects of temperature differences. For example, a balance disclosed in DE 10353414 B3 has at least two temperature sensors which are arranged at different locations, wherein an initial correction value is selected dependent on the difference between the output signals of the two temperature sensors immediately after the power supply is turned on. The two temperature sensors, more specifically the temperature difference measured by them, serve to indirectly determine the length of time during which the device was switched off before being switched back on. Through mathematical computing means, the weight-dependent signal generated by the weighing system is corrected dependent on the length of time elapsed since the device was switched on. This allows a drastic reduction in time from switching on the power supply until the balance reaches its full accuracy. Thus, this proposed solution is directed at the power-up behavior of the balance.
An electronic balance disclosed in GB 2149512A has two measurement sensors which measure factors having a detrimental influence on the weighing result. One of the measurement sensors is for example a temperature sensor which is fastened to the force compensation coil in order to measure the temperature change caused by power dissipation in the compensation coil and to correct the output signal of the measurement transducer of the balance based on the measured temperature change. The data of the temperature sensor are continuously entered into a storage memory of the digital signal-processing unit and stored for a specified length of time. Data originating from different points in time are assigned different weights according to the inertia of the measurement sensor, if they are used for the correction of the output signal of the measurement transducer of the balance. The correction performed is thus of a phenomenological nature.
A balance which is disclosed in DE 29803425 U1 includes a device for the temperature compensation of the zero point signal, wherein in a first step the rate of change dT/dt of the temperature is determined and, if the latter is found to be small enough, the current zero point signal is stored together with the current temperature signal, and wherein, after a sufficiently large number of value pairs have been collected, they serve as the basis for calculating the temperature coefficient of the zero point, which is then used to correct the zero point, for example the zero-load value, of the weighing result.
In a balance described in CH 658516 which is based on the principle of electromagnetic force compensation, the compensating force is generated by means of a current flowing through a coil that is arranged in the air gap of a permanent magnet. As a result of the current, the temperature of the coil is higher than in those components which are arranged farther removed from the coil and whose temperature is determined essentially by the ambient room temperature to which the device is exposed. To compensate for this temperature difference, a temperature sensor is arranged in the interior of the permanent magnet assembly to measure a temperature that corresponds to the increased temperature of the permanent magnet. Based on the measured temperature, the force-measurement signal is then corrected. However, due to the thermal inertia of the permanent magnet, the temperature sensor can respond only slowly to the changes of the ambient temperature. Thus, regardless of the compensation, detrimental influences remain which can introduce errors in the weighing result. Also, the installation, adjustment and inspection of the temperature sensor in the permanent magnet are costly and error-prone. These are the drawbacks of an arrangement where the temperature sensor is located next to or inside a critical component that is directly associated with the actual force measurement.
For a more direct measurement of the heat generation, a concept is proposed in CH 669041 where the temperature sensor is arranged within the windings of the coil. This allows the temperature to be measured at the center of the heat source, for example in the coil windings. Nevertheless, the installation and adjustment of the temperature sensor is still fraught with problems. Furthermore, the measurement value is possibly not representative for the correction of the temperature dependence of the magnetic field, as the latter depends primarily on the temperature of the permanent magnet and only secondarily on the temperature of the coil.
Exemplary embodiments of the present disclosure are directed to an improved method for the temperature correction in a force-measuring device, specifically in a balance, with the aim of achieving a simple and cost-effective operation while simultaneously meeting high standards in regard to measurement accuracy and stability. In particular, the effort is directed at providing a method for the compensation of a temperature difference, where the temperature sensor can be installed, adjusted and inspected in a simple manner. Other exemplary embodiments are also directed to a suitable force-measuring device of a simple, cost-effective and reliable design.
The following observations principally concern the normal operation of a balance. Power-up episodes and malfunctions are not taken into account.
An exemplary embodiment of the present disclosure include a method for the temperature correction of a force-measuring device, specifically a balance, during its normal operation, with the steps: by means of a force-measuring cell, generating a force measurement signal corresponding to the input force; measuring a temperature by means of a temperature sensor that is arranged at a distance from the heat-generating components of the force-measuring device, wherein said temperature corresponds primarily to an ambient temperature to which the force-measuring device is exposed, and generating a temperature signal corresponding to the measured temperature; processing the force measurement signal into a temperature-corrected output signal based on the temperature measurement signal and the force measurement signal; and transmitting the output signal to an indicator unit and/or to a further processing unit. In the processing step, at least one correction parameter serving for the correction of the output signal is calculated from the force measurement signal and the temperature measurement signal by means of an underlying thermodynamic model, wherein the correction parameter represents a temperature difference which exists between a system temperature and the measured temperature and/or between a first system temperature and a second system temperature.
Exemplary methods disclosed herein include a simple, efficient and precise temperature compensation can be performed without the need for a costly and/or error-prone installation of a temperature sensor next to or inside of critical components of the force-measuring device. This method can also eliminate the need to install further temperature sensors at different locations of the force-measuring device. Accordingly, a simple and cost-effective design of the force-measuring device can be achieved. In addition, advantages are gained in regard to costs, reliability and stability of the force-measuring device, as the calculations can be performed through simple means in retraceable steps.
Thus, compared against known devices, exemplary embodiments disclosed herein provide an additional thermal influence factor which significantly affects the force-measurements that is calculated by way of a thermodynamic or physical model. This calculated influence factor is then used for an additional correction of the force measurement signal.
The temperature sensor is arranged at a location of the force-measuring device where the temperature measured by the sensor corresponds primarily to the ambient temperature. Temperatures occurring at or near heat-generating components can be calculated by means of the thermodynamic or physical model. The term “ambient temperature” as used in the present disclosure includes different temperatures of the environment to which the force-measuring device is exposed. For example, the temperature of the atmosphere surrounding the force-measuring device or existing in the space inside a housing of the force-measuring device, as well as the temperature of the surface or table on which the device is standing, or the temperature of the object being measured.
The ambient temperature is always acting on the force-measuring device and on a temperature sensor that is arranged at or inside the force-measuring device. However, the measured temperature in some places can deviate from the ambient temperature, for example if additional temperature factors connected to the operation of the force-measuring device enter into the situation.
The term “system temperature” as used herein refers to a temperature that can be assigned to a system component of the force-measuring device. For example, the temperature of a component or part of a component or of a group of components such as the coil, the permanent magnet, the core of the permanent magnet, the air gap, a lever, the position sensor or its mounting attachment, as well as the force receiver, the force-transmitting mechanism, or the force-measuring cell.
In accordance with an exemplary embodiment, the temperature sensor is arranged so that the measured temperature, and thus the electrical temperature measurement signal, corresponds primarily to the ambient temperature. For example, the sensor can be in direct thermal contact either with the ambient atmosphere or with those components whose system temperature is determined primarily by the ambient temperature. Less suitable is an arrangement where the temperature sensor is close to components whose temperature is determined essentially by operation-related factors of the force-measuring device. Thus, the temperature sensor can be arranged on or near thermally passive or stationary components of the force-measuring cell. However, it can also be placed next to components that are exposed to the environment or located on the outside of the force-measuring device, such as the housing, or it can be attached to movable parts such as the force receiver or the force-transmitting mechanism. Furthermore, the temperature sensor could also be arranged outside of the housing or in the vicinity of the force-measuring device.
This exemplary arrangement has the advantageous result that the temperature sensor can react directly and rapidly to changes of the ambient temperature. The measurement of the ambient temperature is further to a large extent independent of the force measurement and thus disconnected from the influence of the latter, for example from heat generation associated with it. It is of particular advantage that the temperature sensor can be arranged at almost any non-critical locations or components of the force-measuring device, so that the installation, adjustment and/or inspection of the temperature sensor can be performed without a problem.
In principle, the method according to the invention can be used for any corrections of temperature differences in force-measuring devices. Thus, it is possible to correct a temperature difference resulting from heat that is generated, for example, in a sudden surge. On the other hand, it is also possible to correct temperature differences resulting generally from the difference between the ambient temperature and a system temperature or between two system temperatures. The correction parameter CP can thus represent different temperature differences.
Situations with different system temperatures can occur if the ambient temperature acting on the force-measuring device changes at a relatively rapid rate and the system temperatures, because of the thermal inertia of the components, cannot follow the changes, or at least not immediately. The resultant temperature difference thus comes from a change that occurs over a short time interval or instantaneously and therefore manifests itself only during a certain length of time, while the system temperature of a component occurs with a time delay after the jump of the ambient temperature. However, these temperature differences smooth themselves out over time, until a new equilibrium level has been reached in accordance with a certain speed of response.
The speed of response, and thus the development of a time-dependent temperature difference, depends on the thermal characteristics of the components themselves and/or their ties to other components and/or to the ambient environment. These thermal characteristics can be connected to a diversity of thermodynamic factors, for example to the thermal inertia, the heat influx, heat conduction, and outflow of heat, as well as to the mass or the surface of the components, and also to their thermal expansion.
It is of particular advantage that the correction according to the invention is not restricted with regard to the locations in the force-measuring device where it can be used, as the system temperature can be calculated for any location and/or component of the force-measuring device. Accordingly, there are no locally restricted measurement areas as would be the case with temperature sensors whose measurement values may not be representative for the component being measured.
The system temperature can be determined without additional cost for different locations and/or components without the need to install a multitude of sensors. Thus it is also possible to determine very complex forms of temperature distributions in the force-measuring device in a cost-effective manner and use them for the correction. In particular, based on a single temperature measurement made at a non-problematic location it is possible to calculate the temperature difference between two different components of the force-transmitting mechanism as well as the influence that this temperature difference will have on the determination of the force measurement signal.
This further eliminates all error-causing thermal influences that are associated with thermal effects in the mounting attachment of the temperature sensor. For example, adhesive bonding of the temperature sensor to a component normally produces a heat-insulating boundary layer which often leads to errors and/or time lags in the system temperature being measured. Problems of this kind will not occur if the system temperature is determined through a calculation.
Further, by refining the calculations in accordance with the specified levels of accuracy, it is possible to calculate complex and diverse thermal influence factors and apply them to the correction of the force measurement signal.
The term “signal-processing unit” in the present context covers all signal-processing elements that are suitable for the processing of the measurement signals of the force-measuring cell and the temperature sensor, for example analog or digital circuits, integrated circuits, shift registers, processors, computers and the like.
In a first advantageous embodiment of the invention, the correction parameter is calculated by means of a response function and a convolution integral. Thermodynamic phenomena that lead to corresponding temperature differences can thereby be calculated in a compact form and applied to the correction. The correction parameter can be calculated by convoluting the temperature measurement signal with the time derivative of the response function, the force measurement signal or a dissipation power function determined from the force measurement signal. This allows the measured signal, including in particular its profile over a preceding length of time, to be taken into account in the calculation of the correction parameter. The individual steps of this calculation are presented hereinafter in the detailed description of exemplary embodiments.
In principle there are a multitude of time-dependent functions that can be used as response functions, for example exponential functions or polynomials. The response function can be defined in accordance with at least one systematic concept of thermodynamics and/or as a thermodynamic response to an input in the form of a step function or impulse function and/or as a time decay function, in particular an exponential function with a given time constant. A diversity of thermodynamic phenomena such as heat conduction, thermal expansion, or the weakening of a magnetic field with increasing temperature can be represented through a model and used for the calculation of the correction parameter. The calculation can furthermore be adapted to the given conditions of a practical situation, for example by adapting the time constants to the thermal conductivities of the different materials.
In an exemplary embodiment of the present disclosure, the correction parameter is calculated by a recursive method. This makes it possible to calculate the values of complicated formulas, in particular the convolution integral, by means of simple mathematical operations, for example by elementary multiplications, summations and time delay functions. In addition, the storing in memory of numerical values and intermediate results can be strongly reduced. As a further benefit, this recursive method provides largely delay-free, for example real-time signal processing.
The correction parameter can be calculated as a time-dependent quantity which depends on the time profile, in particular the rate of change, of the measured temperature and/or the force-measurement signal. This correction primarily takes care of temperature differences that manifest themselves in a short-term or transient manner. Consequently, a so-called dynamic temperature correction can be made, whereby a high level of measurement accuracy can be achieved even for short-term temperature changes.
Of particular advantage is the possibility that the time dependence can by taken directly into account in the calculation. This delay-free correction of temperature differences is advantageous in comparison to temperature measurements, in which one has to expect errors that are caused by the temperature sensor itself, due to its thermal characteristics, in particular its thermal inertia.
In a further embodiment, the system temperature corresponds to a temperature of a component of the force-measuring device, in particular a force input device, a force-transmitting mechanism, a lever or lever arm, and the output signal is corrected dependent on the thermal expansion of the component. As a result, a temperature difference which, through changes in the component dimensions, injects an error into the force measurement can be effectively corrected.
In principle, a temperature change causes a dimensional change of all components affected. However, normally the dimensions of the affected components will not change at the same speed, as the rate of change depends on the individual characteristics of each respective component. As a result, a change in the force-transmitting behavior, specifically a shift of equilibrium, will occur in these components or parts thereof. This has the effect of introducing an error into the force measurement which, however, can be effectively corrected with exemplary methods disclosed herein.
The particular component can be a lever or lever arm that serves to transmit the applied force to the force-measuring cell. Thus, the correction parameter corresponds to a lengthwise dimension of the lever or the lever arms. Consequently, the effects related to the ambient temperature can be calculated with a simple thermodynamic model, as will be shown in the detailed description.
In another exemplary embodiment, the correction parameter represents at least two of the temperature differences associated, respectively, with different components of the force-measuring device, in particular two opposite arms of a lever. Temperature differences of this kind occur when different components whose thermal characteristics are not matched to each other respond to a short-term or instantaneous change of the ambient temperature. In this case, exemplary methods of the disclosure is especially advantageous as it does not specify a major processing effort involving a plurality of temperature measurement signals and a correspondingly difficult adjustment. In addition, it can eliminate the installation of several temperature sensors, which contributes to a cost-effective design.
It is of particular advantage that exemplary methods disclosed herein can also be used in complex setups with a multitude of different components. With the inventive concept of the correction parameter, a larger number of temperature differences can be represented and used for the correction.
The correction parameter can in particular contain a group of two values which includes a first temperature difference of a first lever arm and a second temperature difference of a second lever arm. This makes it possible to calculate the complex behavior of several time-dependent processes that are superimposed on each other, in this case the different time-dependent behaviors of the two lever arms.
In another exemplary embodiment disclosed herein, the correction parameter is calculated by means of a differential signal representing the difference between two temperature differences. This calculation eliminates the dependence on a further reference temperature, in particular the ambient temperature acting on the force-measuring device, and the temperature differences can be calculated as a single signal, for example the difference signal. This simplifies the handling of the correction parameter, it particular its transfer into further calculations.
In yet another exemplary embodiment according of the present disclosure, the force-measuring device is based on the principle of electromagnetic force-compensation, and the quantity being represented by the correction parameter is a temperature difference that is connected to the process of generating the compensation force. Using this principle, the force-measuring cell generates a compensation force which balances the input force acting on the force-measuring device. Thus, the force-measuring cell always maintains the same position even when the applied weight changes, so that the phenomenon of creep as is associated with known devices, for example the problem of deformation and/or material fatigue, will be entirely absent or manifest itself only to a very minor extent.
The amount of heat released in the process of generating the compensation force generally causes a temperature increase in those components that serve to generate the compensation force. By using exemplary temperature correction methods disclosed herein, correcting this influence now becomes a remarkably simple matter, in that the temperatures or their effects at the critical locations of heat generation are determined by calculation. Consequently, one no longer has to contend with the effort and expense of installation, adjustment and monitoring of temperature sensors on these components, which is particularly advantageous in view of the fact that the access to these components is normally difficult.
In accordance with another exemplary embodiment, the compensation force is generated by means of a coil through which a current is flowing, whereby power is dissipated into heat. The correction parameter is calculated depending on the force-measurement signal, in particular as a function of the power dissipation which can be derived from the force-measurement signal. This power dissipation generates a temperature change in the coil and in the components surrounding the coil. The temperature change, in turn, causes a force effect that changes with time in the form of a so-called load drift. This load drift can now be corrected in a simple and precise manner according to an exemplary method disclosed herein, in that the correction parameter is determined in direct dependence on the heat source, for example the power dissipation taking place in the coil.
This exemplary method is of particular advantage under time-dependent conditions, when the current flowing through the coil is variable with time and its magnitude therefore depends on the force which is acting at the moment. Accordingly, with variations of the current, different amounts of power are dissipated, resulting in different temperatures and thus time variations of the forces acting in the system. With the calculation, these variable conditions can be compensated instantly and in a flexible way.
In a further exemplary embodiment of the present disclosure, the compensation force is generated by the interaction of a coil and a permanent magnet, in which case the system temperature corresponds to a temperature of the coil and/or of the permanent magnet. As both the coil and the permanent magnet participate in the generation of the force, these components are particularly critical in regard to intrusive steps taken to measure the system temperatures.
Since the coil is normally arranged in the air gap of the permanent magnet, a temperature change in the coil will cause a corresponding change of the permanent magnet due to the heat transfer by way of different components of the force-measuring device or through the air in the air gap. A temperature change of the permanent magnet, in turn, affects the magnetic field of the latter and thus also has an effect on the compensation force being generated. Consequently, a constant input load on the force-measuring device leads to a gradual rise in temperature and thus creates the appearance of a time-dependent change of the input load, for example the phenomenon of load drift.
In the process of calculating the system temperature, this heat transfer which takes place between the coil where the temperature changes originate and the magnet where they produce their effect can be calculated with good approximation and thus used for the temperature correction.
In a further exemplary embodiment, at least two corrections of the force measurement signal are calculated, respectively, through at least two different, substantially separate arithmetic modules, wherein in each module at least one of the aforementioned correction parameters is calculated. With this modular concept, the calculation can be set up to deal with a large diversity of temperature effects and can be adapted in a simple way to meet changing specifications.
The exemplary embodiments disclosed herein further relates to a force-measuring device, in particular a balance, with a force-measuring cell generating an electrical force measurement signal representative of the applied force, with a temperature sensor arranged at a distance from the heat-generating components of the force-measuring device, wherein the temperature sensor measures a temperature corresponding primarily to an ambient temperature acting on the force-measuring device and generates a temperature measurement signal representing the measured temperature, and further with a signal-processing unit which is configured to process the force-measurement signal, based on the temperature measurement signal and the force measurement signal, into a temperature-corrected output signal which can be transmitted to an indicator unit and/or to a further processing unit. The signal-processing unit is suitably configured to calculate, by means of a thermodynamic model, a correction parameter serving for the correction of the output signal of the force-measuring device during its normal operation, wherein the correction parameter represents a temperature difference that exists between a system temperature and the measured temperature and/or between a first system temperature and a second system temperature.
In another exemplary embodiment of the force-measuring device, the temperature sensor is in thermal contact with a component, in particular a stationary component, of the force-measuring device, wherein the system temperature of the component is determined primarily by the ambient temperature acting on the force-measuring device. With this arrangement, the influence that the ambient temperature has on the components can be directly taken into account, whereby a precise compensation of these influence factors can be achieved. The temperature sensor can be arranged on the stationary components of the force-measuring cell, for example on the part where the force-measuring cell is fastened. Thus, the temperature sensor can be mounted in a simple manner on a non-critical part of the force-measuring device and the temperature measurement signal can be transmitted to its destination without affecting the force measurement.
Exemplary methods according to the present disclosure can be implemented in a software program which can be executed in a signal-processing unit and which serves to calculate the output signal in accordance with an exemplary embodiment. In this way, it is possible to achieve a high degree of flexibility and the capability to reuse the calculation algorithm in other applications.